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Chemical Technology • January 2015
Minerals Processing
and Metallurgy
As with purple gold, co-
loured platinum intermetal-
lic compounds lend them-
selves to be treated like
gemstones and could be
facetted by using standard
gem cutting equipment
and techniques. Figure
4 shows a jewellery item
with facetted pink coloured
platinum compounds, also
known as Platigem
®
.
Palladium intermetallic
compounds
According to the binary
In-Pd phase diagram [19],
five intermetallic compounds exist, namely In
3
Pd, In
3
Pd
2
,
InPd, InPd
2
and InPd
3
. The In-Pd intermetallic compound
with composition 50 at.% (48 wt%) palladium and 50 at.%
(52 wt%) indium produces a purplish-pink colour. More
than 95 % of incident light is reflected by gold in the infrared
and longer wavelength range of visible light. At energies
higher than 1,9 eV, the reflectivity falls off rapidly with
diminishing wavelength. The yellow colour of gold results
from its strong absorption of light above energies of about
2,3 eV. The metal reveals the complementary colour of the
absorbed frequencies. With PdIn, the absorption occurs
at lower energies, and the colour of the compound then
appears as purplish pink.
Coatings
PtAl
2
Platinum modified aluminide coatings have been used for
several decades as diffusion barrier coatings in aircraft
and industrial gas turbines. These coatings provide both
improved high temperature oxidation and hot corrosion
resistance. Hot corrosion occurs in gas turbines due to the
presence of contaminants, such as NaCl, Na
2
SO
4
and V
2
O
5
in the gases, which form molten deposits damaging the
turbine blades [27].
The platinum modified nickel aluminide coatings can
exist in two forms depending on how the coatings were
formed. Figure 5 [26] shows the two forms, where (a)
indicates the two phase PtAl
2
+ (Ni-Pt-Al), and (b) a single
phase (Ni-Pt-Al) coating. Platinum is initially deposited onto
the nickel-based superalloy via electroplating, whereafter
it is heat treated under a protective atmosphere. The heat
treatment conditions influence the formation of a single- or
two phase microstructure. Subsequent aluminising results
in the platinum modified NiAl coating.
The advantages that platinum offers in barrier diffusion
coatings can be summarised as follows [27]:
Platinum:
• improves the high temperature oxidation resistance
by delaying transformation of
β
-NiAl into
γ
1
-Ni
3
Al in
aluminides. The life of the diffusion coating is depleted
when all
β
-NiAl has transformed into
γ
1
-Ni
3
Al.
• acts as a catalyst promoting the reaction between
aluminium and oxygen.
• improves the adhesion between the coating and
substrate.
• suppresses deleterious spinal formation.
• retards the diffusion of certain refractory elements
to the coating-Al
2
O
3
interface providing improved
isothermal oxidation resistance.
AuAl
2
, AuIn
2
and AuGa
2
Supansomboon
et al
[38] prepared AuAl
2
coatings by
vacuum deposition onto heated substrates. The coloured
coatings varied in colour from dark-silver to light purple,
whereas the transmission colours of these coatings varied
in colour from light to dark greenish-brown. The colour
observed by the human eye was dependent on the texture
of the substrate, the crystallized microstructure and the
coating thickness affected the transmission colours. The
potential use of AuAl
2
as a spectrally selective coating on
architectural glass was explored, but found to be inferior to
that of gold in terms of selective attenuation of the infrared
radiation. Furrer
et al
[10] found that the light purple colour
for AuAl
2
coatings is due to point defects in the film resulting
from the deposition method. The intense purple colour can
be obtained by heat treating the coating at 350˚C.
Studies by Keast
et al
[16] indicated that PtAl
2
and AuAl
2
coatings have dielectric functions suitable for sustaining
localized plasmon resonances as verified with EELS and
reflectivity measurements. The results suggested that the
PtAl
2
compound is a better candidate for the development
of strong localised surface plasmon resonances compared
to AuAl
2
.
In a project funded by the European commission on
surface engineering of the colour effect for gold alloys, Klotz
[17] found that the electroplating/annealing process was
very successful for producing AuIn
2
layers, whereas surface
cladding worked well for both AuGa
2
and AuIn
2
, and liquid
metal dip-coating for AuGa
2
(see Figure 6).
Figure 4: A photograph of
Platigems (facetted pink
PtAl
2
+Cu) in a jewellery piece
(Mintek brochure)
.
(a)
(b)
Figure 5: Microstructures of Pt modified aluminide coatings on
nickel-based superalloy where (a) two phase PtAl
2
+ (Ni-Pt-Al),
and (b) single phase (Ni-Pt-Al) coating [26].
Figure 6: Gold dip-coated with blue AuGa
2
[17].